The disadvantages of pilot signals include additional loading for any circuit which uses the pilot signal, the provision of a slot in the frequency spectrum for a pilot signal where it does not interfere with other signals, and where the pilot signal is used to characterize changes in a transmission path, the characterization is strictly speaking only applicable to the frequency region occupied by the pilot signal.
The present invention is particularly useful in the removal of distortion in broadband linear amplifiers such as are described in U.S. Pat. Nos. 5,334,946 and 5,157,345. Another method of reducing distortion in such amplifiers is described in U.S. Pat. No. 4,580,105 assigned to A.T & T (inventor Myer). The correction method of this U.S. Pat. No. 4,580,105 suffers from the above mentioned disadvantages and a further disadvantage in that the pilot signal restricts channel usage within the amplifier where a multi-channel input is applied to the amplifier. Previously mentioned U.S. Pat. No. 5,334,946 and 5,157,345 overcame these problems by avoiding the use of a pilot signal but a further problem exists in the particular situation where it is required to combine a large number of input channels for application to the broadband linear amplifier. Such an amplifier is usually used as the final power amplifier of a radio transmitter and for this reason it must be near a transmission antenna. The antenna is usually situated on the top of a building where rentals are high. A large number of input cables, one for each input channel, for example 30 to 100, have to be run to the top floor of this building. In addition some circuits have to be duplicated for each channel near the amplifier and high rental space is required for this purpose.
Another important application of the invention is in radio repeaters, especially for cellular radio, where a repeater for a "hole" in coverage, for example in a tunnel, receives signals at a low level and transmits at a high level on the same frequency. Instability is likely in such repeaters but can usually be avoided by using directional antennas and positioning transmit and receive antennas as far apart as possible. However in many situations one omnidirectional antenna is required and suitable spacing is inconvenient. A similar application which uses a derivative of the technique is in frequency translating repeaters. An advantage then gained is the elimination of the complex filtering otherwise required in these systems.
In code-division multiple access (CDMA) systems each channel employs a spread spectrum signal and the channel signals are added before transmission. U.S. Pat. No. 4,962,507 describes a system in which a pilot signal for transmitting a timing reference is also transmitted in spread spectrum form.